Lift trucks are well-known vehicles for handling loads. Typically, the load is carried by a pair of forwardly projecting forks that are attached to the carriage of the lift truck. The carriage is driven for lifting and lowering the forks.
Loads moved by lift trucks are often placed on pallets. To relocate loaded pallets, the lift truck is moved so that the forks are inserted through pockets in the pallets. The forks are then lifted and the pallet is moved.
Fork lift attachments that allow a lift truck to simultaneously handle two pallets are presently available. Such attachments (hereafter referred to as dual pallet fork attachments) generally include two outer forks and two inner forks. The inner forks are located between the outer forks. A hydraulic control system is used for laterally spreading the forks so that the forks engage the pockets of two juxtaposed pallets. Alternatively, the inner and outer forks may be moved into a position for use as a single fork pair to carry a single pallet.
In moving two pallets, it is often desirable to spread apart the pallets. For example, after two loaded pallets are moved into a trailer, the loads are spread apart against the walls of the trailer so that the overall load is evenly distributed and stabilized within the trailer.
The forks of dual pallet fork attachments are mounted to a guide assembly that guides the lateral movement of the forks. The hydraulic control systems and guide assemblies of conventional dual pallet fork attachments are designed so that only the outer forks are directly driven by the hydraulic system. The inner forks are coupled to the outer forks to slide outwardly when the outer forks are driven outwardly (that is, when the attachment is used for moving two juxtaposed pallets). As the forks are spread to engage two pallets, the maximum distance between the inner forks is established by mechanical stops that halt the outward sliding of the inner forks. The outer forks move outwardly beyond the stops into a position where an outer fork and an adjacent inner fork can engage one pallet, and the other outer fork and adjacent inner fork can engage the other pallet.
As the outer forks are driven toward one another (for example, when the attachment is to be used for handling a single pallet), they push the inner forks inwardly, until the inner forks abut mechanical stops that define the minimum distance between the inner forks.
In instances where two pallets are to be moved by the attachment, it is sometimes necessary to change the position of the inner forks relative to the outer forks. For example, one of two juxtaposed pallets may have a somewhat dissimilar pocket configuration relative to the other pallet. Consequently, the distance between the innermost pockets (that is, the pockets that receive the inner forks) may be greater or lesser than the maximum distance between the inner forks as defined by the mechanical stops mentioned above. As a result, the innermost forks must be moved inwardly or outwardly to align with the inner pockets. With the dual pallet fork attachments of the type just described, any such changes in the position of the inner forks (while the outer forks remain spread to engage the outermost pockets) must be performed manually by the lift truck operator.
As noted, the inner and outer forks are mounted to a guide assembly that supports and guides lateral movement of the forks. The guide assembly is generally disposed between the vertical legs of the forks and the lift truck carriage. The structural components of the guide assembly must be strong enough to resist the moment of force that is generated when the forks are loaded. It is also desirable, however, to minimize the size of the guide assembly components so that the forks may be placed as near as possible to the lift truck carriage because the load carrying capacity of the lift truck is reduced as the distance between the forks and carriage is increased. The distance between the forward surface of the lift truck carriage and the forward surfaces of the vertical legs of the forks is known as the lost load distance. In short, it is desireable to minimize the lost load distance. Moreover, reducing the size of the attachment components reduces the weight of the attachment, hence increasing the carrying capacity of the attachment.
The versatility of a dual pallet fork attachment is enhanced when the attachment includes a mechanism known as a side shifter. Side shifters provide for simultaneous lateral movement of both pairs of forks relative to the carriage. Side shifters eliminate a substantial amount of lift truck movement that would otherwise be required to achieve such movement of loads or forks.